1. Field of the Invention
The present invention relates to a thin film transistor using a silicon active layer.
2. Description of the Related Art
Since display elements such as electroluminescence, a light-emitting diode, a plasma, fluorescence, and a liquid crystal can provide flat panel type display devices, these devices have been demanded to be applied as special-purpose display devices such as terminal display devices of measuring instruments, office machines, and computers. Of these devices, an electroluminescent or liquid crystal display device using a switching element matrix array consisting of thin film transistors has received a great deal of attention as a display device having low power consumption and low cost.
As materials of these switching transistors, single-crystalline, polycrystalline, and amorphous Si, CdSe, Te, CdS, and the like are used. Since a thin film technique in a low-temperature process can be applied to a polycrystalline semiconductor and an amorphous semiconductor of these materials, an active matrix element consisting of switching transistors can be formed on even a substrate as of glass required to be treated at a relatively low temperature, and a large, inexpensive display device has been available.
A thin film transistor (TFT) of this type is described in a literature, e.g., Published Unexamined Japanese Patent Application No. 102560/83.
In the above conventional TFT, amorphous silicon (a-Si) films serving as an active layer and an ohmic contact layer cannot be continuously formed because the steps of forming and patterning a protection film (contributing to an etching process for forming source and drain electrodes separated from each other) interrupt this. As a result, when the protection film is patterned, the active layer is also etched, and a junction between the active layer and the ohmic contact layer is degraded. Thus, a good ohmic contact cannot be obtained, and the TFT characteristics are degraded.
In addition, when an active layer and an ohmic contact layer are continuously formed without interposing a protection film therebetween, and the ohmic contact layer is patterned, the active layer is required to have a large film thickness because the active layer is etched when the ohmic contact layer is patterned. For this reason, a process for forming the films requires a long time, or the TFT is always set in an ON state. Therefore, the reproducibility of the TFT characteristics is degraded, and productivity and a yield of products are decreased.
As described above, in the conventional TFT, the active layer and the ohmic contact layer cannot be continuously formed, and the active layer is damaged when the protection film is formed. Therefore, perfect ohmic contact is difficult to be obtained between the active layer and the ohmic contact layer, thereby degrading the TFT characteristics.